In the case where limbs are badly damaged or large amounts of bone must be surgically removed, e.g. in the case of treatment of bone cancer, there is a need for prostheses which replace a large part of the natural bone and which can be assembled by the surgeon to meet a wide range of conditions. The present invention provides a modular system and components thereof for constructing effectively customised prostheses to meet a range of such surgical requirements. Typical requirements may be, for example, to replace the proximal or distal femur. Existing modular prostheses for the purpose such as, for example, those in U.S. Pat. No. 4,623,349, U.S. Pat. No. 4,963,155 and U.S. Pat. No. 6,048,365 generally allow customisation but generally do not accommodate for significant differences between patients and do not allow for effective, cost-effective and efficient tailoring of the prosthesis to the needs of a patient.